Why diffraction works at sharp edges?

Question

While introducing Diffraction, physics textbooks say that this effect (Diffraction) is observed distinctively when the light is passed through a very small opening, the length or diameter of which is comparable with the wavelength of the light. Later, they also end up saying that Diffraction is also observed at sharp edges (which are not associated with any small opening).

Following are some questions:

1. If Diffraction can happen at some edges also, then the wavelength of light shouldn't matter. Why does wavelength matter in diffraction, and how is it related to edges?

2. Diffractions at edges make me question the fundamentals of diffraction that why and how it happens? How the light can bend at the sharp edges, and why?

3. I understand the explanation given by Huygens principle for small openings; diffraction of light at sharp edges with the help of Huygens principle is not understandable.

Kindly help.

Answer 1

Imagine a plane wave passing an edge.
Geometrical optics predicts that after the edge is passed there is a region where plane waves exists and a region (shadow) were there are no waves and that there is a sharp divide between the two regions.

In practical terms how can that be so?
As an example let's choose water waves.
The conclusion from geometric optics is that a moving (due to the wave motion) water molecule adjacent to a water molecule in the shadow region does not make the water molecule in the shadow region move even though there are intermolecular forces between them.
This is impossible and so the water molecules in the shadow region, which are communicating with one another via inter-molecular forces, are moving and this effect we call diffraction.

Huygens principle is a mechanism for predicting what happens when a wave passes an obstacle and up to a point, it works.

Why does wavelength matter in diffraction, and how is it related to edges?
The size of the region over which diffraction is observable is on the scale of the wavelength of the wave.
If that region is small compared with the resolution of the detector the diffraction will not be observed as will also be the case if the intensity of the non-diffracted wave "swamps" the diffracted wave.

Answer 2

Here's another way of visualizing Farcher's point. We take the case of diffraction happening at the edges of a slit in a thin razor blade:

When struck by an incoming plane wave, the corner at the edge of the slit reflects the plane wave into an outgoing spherical wave train centered at that corner. Part of that spherical wave wraps around the corner and propagates its way into the space on the other side of the razor. This happens on both sides of the slit. Now you have the opportunity for phase cancellation within the space downstream of the slit.